1. Field of the Invention
The present invention relates to laser-sustained plasma illuminator systems. More particularly, the invention relates to systems and methods for compensating for optical aberrations to optimize plasma performance and UV light collection.
2. Description of Related Art
Plasmas sustained by lasers have shapes defined by the laser light intensity distribution near the laser focus. The laser light intensity distribution may be a function of optical aberrations (e.g., how well the light is focused in the plasma cell). Many optical aberrations present in typical laser-sustained plasma illuminator systems are aberrations introduced by an enclosure (e.g., a bulb) used to contain the gas and the plasma. Such bulb-introduced aberrations may be significant optical aberrations, especially for plasmas sustained by lasers operating in the near IR range (wavelengths of about 1000 nm). These significant optical aberrations may result in large size plasmas, the inability to control the bulb envelope, and/or irreproducible plasma shapes.
FIG. 1 depicts different plasma shapes resulting from various optical abberrations of a pump beam in different bulbs. Shape 100 results from a pump beam with significant aberrations. These significant aberrations produce a football shaped plasma for shape 100. Shape 102 results from a pump beam with less aberrations. These fewer aberrations may produce a spherical shaped plasma for shape 102. Shape 104 results from a pump beam with the fewest aberrations. Shape 104 may be the smallest and brightest plasma shape (e.g., a cylindrical plasma shape) of the three shapes depicted in FIG. 1 because of the fewest aberrations.
Aberrations may become particularly large when a high NA (numerical aperture) is used for pumping the plasma. Large pump laser NAs are used as light sources in many current laser-sustained plasma illuminator systems. U.S. Pat. No. 7,705,331 to Kirk et al., which is incorporated by reference as if fully set forth herein, describes an example of a high NA system. FIG. 2 depicts an example of a laser-sustained light source with a high NA. Light source 200 may include laser 202, turn mirror 204, cold mirror 206, homogenizer 208, filters 210, ellipse 212, and enclosure 214. Enclosure 214 may be, for example, a bulb. Ignition cable 216 may be coupled to enclosure 214. Plasma 217 may be generated inside enclosure 214 at or near a focal point of ellipse 212. As shown in FIG. 2, light from laser 202 (e.g., light 218) may be reflected off ellipse 212 and focused in the middle of enclosure 214 at plasma 217. Broad-band UV light (e.g., light 220) from homogenizer 208 may be reflected by cold mirror 206, reflected off ellipse 212, and focused in the middle of enclosure 214 at plasma 217. Light passing through enclosure 214 may be used to excite and/or sustain plasma 217 inside the enclosure. Plasma 217 inside enclosure 214 may provide light for illumination of a specimen for a process performed on the specimen (e.g., an inspection process performed on the specimen). As shown in FIG. 2, light passing through enclosure 214 may have a high NA.
In addition to the aberrations introduced by the enclosure itself, the refractive index of the gas inside the enclosure is another source of aberrations in the system. Gas related aberrations may be especially significant in high-pressure enclosures. FIG. 3 depicts images taken of a bulb at different pressures of Xe (xenon) in the bulb. As shown in FIG. 3, aberrations seen from the bulb increase with increasing pressure.
U.S. Pub. Pat. Appl. Nos. 2007/0228288 and 2007/0228300 to Smith, each of which is incorporated by reference as if fully set forth herein, disclose one method of compensating for aberrations introduced by the refractive index of the walls of the enclosure by modifying the shape of the reflector (e.g., a reflective ellipse). Modifying the shape of the reflector, however, can only account for aberrations from reproducible enclosure shapes. Modifying reflector shapes for each individual enclosure shape and/or different fill pressures is difficult to impractical to implement for most laser-sustained plasma illuminator systems.